The present invention relates to the fabrication of microstructures on a substrate and, in particular, to processes for fabricating masks for the fabrication of microstructures, such as emitter tips for field emission displays, on a substrate.
The fabrication of micron and sub-micron structures or patterns into the surface of a substrate typically involves a lithographic process to transfer patterns from a mask onto the surface of the material. Such fabrication is of particular importance in the electronics industry, where the material is often a semiconductor.
Generally, the surface of the substrate is coated with a resist, which is a radiation sensitive material. A projecting radiation, such as light or X-rays, is then passed through a mask onto the resist. The portions of the resist that are exposed to the radiation are chemically altered, changing their susceptibility to dissolution by a solvent. The resist is then developed by treating the resist with the solvent, which dissolves and removes the portions that are susceptible to dissolution by the solvent. This leaves a pattern of exposed substrate corresponding to the mask.
Next, the substrate is exposed to a liquid or gaseous etchant, which etches those portions that are not masked by the remaining resist. This leaves a pattern in the substrate that corresponds to the mask. Finally, the remaining resist is stripped off the substrate, leaving the substrate surface with the etched pattern corresponding to the mask.
Another method useful for fabricating certain types of devices involves the use of a wet dispense of colloidal particles. An example of this technique is described in U.S. Pat. No. 4,407,695, the disclosure of which is incorporated herein by reference. With the wet dispense method, a layer of colloidal particles contained in solution is disposed over the surface of a substrate. Typically, this is done though a spin coating process, in which the substrate is spun at a high rate of speed while the colloidal solution is applied to the surface. The spinning of the substrate distributes the solution across the surface of the substrate.
The particles themselves serve as an etchant, or deposition, mask. If the substrate is subject to ion milling, each particle will mask off an area of the substrate directly underneath it. Therefore, the etched pattern formed in the substrate surface is typically an array of posts or columns corresponding to the pattern of particles.
Although the wet dispense method has some advantages over the lithographic process, it has its own deficiencies. For example, the spinning speed must be precisely controlled. If the spin speed is too low, then a multi-layer coating will result, instead of the desired monolayer of colloidal particles. On the other hand, if the spin speed is too high, then gaps will occur in the coating. Further, owing to the very nature of the process, a radial nonuniformity is difficult to overcome with this method.
Another problem with colloidal coating methods is that they require precise control of the chemistry of the colloidal solution so that the colloidal particles will adhere to the substrate surface. For example, if the colloidal particles are suspended in water, the pH of the water must be controlled to generate the required surface chemistry between the colloidal particles and the substrate. However, it is not always desirable to alter the pH or other chemical properties of the colloidal solution. Also, if the colloidal solution fails to wet the surface of the substrate, the particle coating may not be uniform.
In addition, wet dispense methods tend to be expensive and prone to contaminating the substrate.